LPG Boiler cutting in/out too frequently when heating water only

[rhino666]

I have a Glow.Worm 30HXI boiler running on LPG. It has been excellent and no apparent problems or fault codes.
Currently not running heating; just used to heat up water for an hour, a couple of times a day.
I noticed a bit of a gas smell from the external vent and the boiler firing up and then cutting out after a short time when it was heating water earlier today.
On inspection it repeatedly fires up when temperature on boiler shows a little over 30 degrees, runs for perhaps 10 seconds that raises the temperature to over 70 degrees and then cuts out.
The temperature shown on the boiler display very quickly drops back to the 30 or so degree level where the boiler cuts in again.
This fast cycling indicates that there is a problem to me that I hope someone can identify.

Thanks

Rob

 

[Chuck]

This fast cycling indicates that there is a problem to me that I hope someone can identify.

It suggests that the circulation of system water around the loop that serves the domestic hot water tank is not fast enough. Could be a faulty pump, faulty valve or some sort of silt build up (particularly if it is a 'gravity circulation' system without a pump.

Another, less likely, possibility is that the tank thermostat is set too high or has failed and is calling for heat when the tank is too hot for heat to be transferred in effectively.

A few more details about they system and maybe a couple of pictures might help us be more specific.

 

[rhino666]

It suggests that the circulation of system water around the loop that serves the domestic hot water tank is not fast enough. Could be a faulty pump, faulty valve or some sort of silt build up (particularly if it is a 'gravity circulation' system without a pump.

Another, less likely, possibility is that the tank thermostat is set too high or has failed and is calling for heat when the tank is too hot for heat to be transferred in effectively.

A few more details about they system and maybe a couple of pictures might help us be more specific.

Thank you for your reply.
I hope the attached pics help to clarify the system.
It is closed with a hot water tank that seems to provide hot water for baths, etc without issue.
No fault codes.
No expert on these systems but second photo shows a pump and diverter valve? that I believe indicates pumped flow to the hot water tank. Boiler is ground floor and tank on first floor.
Just checked central heating as not run since last test a month or so ago. Switched on for half an hour and temperature at boiler display gently climbs from 24 degrees to somewhere around 70 degrees. Cuts out and back in as expected.
Hot water tank is only 114 litres, heated up twice a day for an hour. Perhaps this heating time needs reducing?
Tank stat set at 55 degrees; not touched since we moved in over 9 years ago.

Thanks

 

[Chuck]

Just checked central heating as not run since last test a month or so ago. Switched on for half an hour and temperature at boiler display gently climbs from 24 degrees to somewhere around 70 degrees. Cuts out and back in as expected.

That suggests to me that the pump is okay. How does it behave when the hot water is rapid cycling? Also, what's the diverter valve doing?

Is there another zone valve for the hot water? It may be near the tank controlled by the thermostat.

 

[rhino666]

That suggests to me that the pump is okay. How does it behave when the hot water is rapid cycling? Also, what's the diverter valve doing?

Is there another zone valve for the hot water? It may be near the tank controlled by the thermostat.no

There is no other zone valve, just the Honeywell V4073A1039 3 port mid position valve. I found a test writeup on it and it appear to work perfectly. The pump runs very quietly, so checked that in operation with a finger on the casing to pick up vibration.
It's been cold here today so a good test for the hour long hot water routine this evening. Pump and boiler straight on as scheduled and boiler temp up from less than 30 degrees to probably 70 degrees in under 15 minutes. Pump continued running when max temp reached and boiler cut in after quite a long time when temperature dropped back. All seemed perfectly normal. Strangely at the end of the hour program temp dropping at around 50 degrees, no boiler as expected but also no pump! I would have expected the pump to run for the whole time. Even without the pump running the temperature shown dropping very slowly.
Can't understand what could possibly have caused the boiler temperature to fly up and down in seconds with just the hot water program the other night.
The 30HXI is quite a powerful boiler and the 114 litre tank is a very small cylinder I think. Have reduced the hot water program to half an hour, twice a day to see if that works for us and hopefully strange recycling was a one-off.

 

[Chuck]

Strangely at the end of the hour program temp dropping at around 50 degrees, no boiler as expected but also no pump! I would have expected the pump to run for the whole time.

What I'd expect to happen is for the boiler to shutdown but the pump to continue running for a short time, say 30s. (This is the 'overrun' time, which ensures the boiler heat exchanger doesn't surge in temperature.) During this period the water in the loop equilibrates with the temperature in the cylinder, so the boiler thermometer ends up at ca 50°C.

Can't understand what could possibly have caused the boiler temperature to fly up and down in seconds with just the hot water program the other night.

Evidently the pump failed to start. Most likely cause if you have a conventional pump is a failing 'starter capacitor'. This problem would explain intermittent starting.

 

[rhino666]

Evidently the pump failed to start. Most likely cause if you have a conventional pump is a failing 'starter capacitor'. This problem would explain intermittent starting.

Makes a lot of sense. Does not appear to be any kind of fault code that would be generated by this.
I imagine the starter capacitor is on the main PCB, part number 2000802731.
Appreciate the advice; thanks very much.

 

[Chuck]

I imagine the starter capacitor is on the main PCB, part number 2000802731.

No, it would be part of the motor.

 

[rhino666]

No, it would be part of the motor.

That's a shame; I thought I may have found it - see blowing capacitor top in first photo.
Circulating pump is a Grundfos 96281473 super electric 15-60 130. Please see second attached photo. Any way of testing the capacitor without blowing my multimetre?

 

[Chuck]

That's a shame; I thought I may have found it - see blowing capacitor top in first photo.
Circulating pump is a Grundfos 96281473 super electric 15-60 130. Please see second attached photo. Any way of testing the capacitor without blowing my multimetre?

Bulging capacitors are not good but since the PCB is part of the the boiler control circuitry we're not allowed to give advice about repairs of this type on this forum. You should contact a Gas Safe Registered Engineer if you think that's where the problem lies.

For the pump motor, which is not part of the boiler and so we can discuss here, I'd monitor the mains feed to it during the fault condition. If the feed is steady but the motor is not turning something's up with the pump. To be more specific, if it takes a while to start, sometimes works and sometimes doesn't, hums more than you'd expect, etc. it's probably the capacitor. A simple test of the capacitor would involve isolating the pump, ensuring the capacitor is safely discharged, disconnecting it and then using a capacitance meter (or scale if your multimeter has one) to see if it's in spec or not. Capacitors are not expensive it might be easiest check by substitution if you don't have the necessary test gear.

For safety, capacitors to be properly sized and rated. It's ensential to get the right replacement. If you're not sure just replace the pump, it's not worth risking a fire.

P.S. It may not be the pump/capacitor. It's just the most probable cause that seems to me fits the symptoms you've described. Others may have alternative suggestions.

 

[rhino666]

I'm sorry to revisit my thread but the hot water issue continues. Main boiler PCB replaced and pump thoroughly checked; no change in this issue. Tonight I stayed with the boiler for 50+ minutes to document the hot water scenario.
Times are stop watch mins/secs............
0.00 15 degrees pump fired straight up and ran consistently and quietly throughout.
3.00 Steady climb to to around 60 degrees then all over the place, back to 30, hovered and then climbed quickly
3.45 cut out at 72 degrees
4.22 fired up at 62
5.20 cut out at 72
5.50 fired up at 55
6.04 cut out at 72
6.36 fired up at 55
7.00 cut out at 72
7.22 fired up at 53
8.20 cut out
8.51 fired up
9.12 cut out
9.43 fired up
10.01 cut out
10.32 fired up
10.55 cut out
11.24 fired up
12.21 cut out(settling)
16.00 54
18.00 52
22.00 50
25.00 49
32.20 48
39.30 47
49.00 46 fired up
51.15 72 cut out
Steady drop as before(stabilized)

To summarise the boiler and pump appear to be working perfectly. The problem is that the circulating water is initially heating up and cooling down too quickly, hence the nine fire ups and cut outs where there would ideally be just one. Boiler and hot water tank were installed in 2006. There is very little hard water deposit down here in South Devon but it seems to me that the issue may be a failure of the heat to effectively transfer from the coil system in the tank to the contents. I think this is causing the limited boiler heated circulatory water to have very unstable temperature until the hot water tank contents finally get up to temperature and stop a fast cooling of the circulating water. The Grundfos 16/60, 130 pump(pic earlier in thread) is set to speed three(of three). Quite a lot of radiators in the house so works well for central heating. Perhaps the hot water system with small 114 litre tank would work better if the pump was set at one or two? If anyone can think of anything else, input much appreciated.

 

[Chuck]

I'm sorry to revisit my thread but the hot water issue continues.

What model of hot water tank do you have?

 

[John.g]

I have a standard 0.62 M2 coil heating ~ 100 litres of a twin coil 150 litre cylinder with a oil fired boiler so I would think similar coil performance, my boiler runs at 70C.
I took a few readings years ago after draining the HW cylinder until full of cold water at 13C and heated it to 55C, took 65 minutes (20 kw boiler cycling on/off since it can't modulate)
Just to give you some idea of the power required/absorbed at some different cylinder temperatures.
7kw @ 13C
6kw @ 20C
4kw @ 40C
3kw @ 50C
2.5kw @ 55C

I would suggest draining off all the hot water until you have a cylinder full of cold water and fire up the boiler and see how it performs.
Presuming you have a modulating gas boiler down to maybe 6 kw then it should fire continuously for anything between ~ 2 and 21 minutes (see below) to reach 30C from cold at 10C, the heating demand is < 5 kw after this but shouldn't cycle excessively even up to 55C (2.5 kw).
If the boiler is of the modulating type then it should run at ~ 6.5 kw up to cylinder temp of 30C and as it requires 2.3 kwh to heat 100 litres from 10C to 30C then the boiler should run continuously for ~ 21 minutes but if not a modulating type should still run) for a minimum of 2 minutes before cycling begins.
Now I accept that it appears you have a different problem but carrying out the above
may just give you a clue as to the problem.
Was the boiler flow temperature probe changed?.

Just read that the boiler can modulate to ~ 5.5kw so should definitely run based on the above for 20/25 or so minutes initially with a cold cylinder until 30C.
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You may have already checked but.....
Since you have a lot of radiators with a mid position valve which means that you can heat the CH & HW together its possible that a balancing wheel (gate) valve is fitted in the coil return, just check for this and shut it fully, then open it fully (3 to 4 full turns). If this makes a difference then you can fully shut it again and open it 3/4 to 1 full turn.
If or if not carrying out the above suggested cold test and if the boiler starts acting up switch on the CH+HW and assuming no boiler antics see if the HW cylinder is heating up as well as the rads, also feel the coil return, it should be fairly hot but way cooler than the flow.

 

[rhino666]

What model of hot water tank do you have?

Please see attached photos. Tank seems to have been installed at the same time as the Glowworm 30HXI boiler in 2006.
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I have a standard 0.62 M2 coil heating ~ 100 litres of a twin coil 150 litre cylinder with a oil fired boiler so I would think similar coil performance, my boiler runs at 70C.
I took a few readings years ago after draining the HW cylinder until full of cold water at 13C and heated it to 55C, took 65 minutes (20 kw boiler cycling on/off since it can't modulate)
Just to give you some idea of the power required/absorbed at some different cylinder temperatures.
7kw @ 13C
6kw @ 20C
4kw @ 40C
3kw @ 50C
2.5kw @ 55C

I would suggest draining off all the hot water until you have a cylinder full of cold water and fire up the boiler and see how it performs.
Presuming you have a modulating gas boiler down to maybe 6 kw then it should fire continuously for anything between ~ 2 and 21 minutes (see below) to reach 30C from cold at 10C, the heating demand is < 5 kw after this but shouldn't cycle excessively even up to 55C (2.5 kw).
If the boiler is of the modulating type then it should run at ~ 6.5 kw up to cylinder temp of 30C and as it requires 2.3 kwh to heat 100 litres from 10C to 30C then the boiler should run continuously for ~ 21 minutes but if not a modulating type should still run) for a minimum of 2 minutes before cycling begins.
Now I accept that it appears you have a different problem but carrying out the above
may just give you a clue as to the problem.
Was the boiler flow temperature probe changed?.

Just read that the boiler can modulate to ~ 5.5kw so should definitely run based on the above for 20/25 or so minutes initially with a cold cylinder until 30C.
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You may have already checked but...
Since you have a lot of radiators with a mid position valve which means that you can heat the CH & HW together its possible that a balancing wheel (gate) valve is fitted in the coil return, just check for this and shut it fully, then open it fully (3 to 4 full turns). If this makes a difference then you can fully shut it again and open it 3/4 to 1 full turn.
If or if not carrying out the above suggested cold test and if the boiler starts acting up switch on the CH+HW and assuming no boiler antics see if the HW cylinder is heating up as well as the rads, also feel the coil return, it should be fairly hot but way cooler than the flow.

John, I have an instinctive feeling you have nailed it if as I suspect this cycling is created by excessive flow speed upsetting heat permeation/diffusion. There is a balancing gate valve fitted; see picture and it is very possible that I upset it some years ago when tackling a series of leaks in the rotted copper pipe. Shut every valve off in sight and then wound fully open when issue with pipes finally resolved. It takes 5.5 turns to wind down fully, so wound down fully and then opened just one full turn. That should slow the flow sufficiently for a test later. The temperature probe has not been changed in the nine years we have lived here but reasonably sure it is working correctly.

 

[Brambles]

Follow John’s advice, but also measure the dhw coil flow and return temps at the cylinder. On Dhw only they should be very close to the boiler flow and return temps.

If they are not look at the by pass valve arrangement - is it acting at the path of least resistance - or acting properly and you have a restriction in the DHW coil circuit - which could be at the valve

 

[John.g]

I was hoping that the balance valve was throttled too much, I have never throttled mine as IMO it gives a faster cylinder warm up, I also have the DHW timed to come in for 1.5 hrs in the morning before requirement for CH, in the evening its programmed to be open all the time (via cylinder stat) from 330 until system shutdown at midnight, it doesn't seem to have any noticeable effect on the CH but of course the house is fully warm then.
I think there is yet another regulation or other now that balancing valves are not to be fitted??.

 

[Chuck]

It takes 5.5 turns to wind down fully, so wound down fully and then opened just one full turn. That should slow the flow sufficiently for a test later.

If you've been operating with it only open one turn, that's probably your issue. Open the valve fully and close it half a turn (to stop it seizing).

These valves are virtually redundant these days. They were needed in the past when the hot water was not on it's own independent zone and the heat exchangers in tanks had a lower capacity than do these days.

 

[rhino666]

If you've been operating with it only open one turn, that's probably your issue. Open the valve fully and close it half a turn (to stop it seizing).

These valves are virtually redundant these days. They were needed in the past when the hot water was not on it's own independent zone and the heat exchangers in tanks had a lower capacity than do these days.

No Chuck, the balancing valve was always previously operating fully open. I thought this issue may have been too fast a flow through the small tank where contrasting powerful flow required for CH on a 250m2 house. Have reset valve fully opening and then wound back in half a turn as advised. Just to test the hot water issue have turned Grundfos back from highest speed setting to lowest speed to see if this makes any difference; will report back later.

 

[John.g]

IMO if circulation problems on HW only more likely to be too low, not likely with the balancing valve fully open but a air lock in the coil can cause havoc, there should really be a air vent as close to the coil entry to the cylinder as possible otherwise one would have to slacken/break the connection and pull the pipe back very slightly to check/release any air, however if the coil is circulating and heating the cylinder with both HW & CH on then unlikely as lukewarm water is normally a sign of air.

 

[rhino666]

IMO if circulation problems on HW only more likely to be too low, not likely with the balancing valve fully open but a air lock in the coil can cause havoc, there should really be a air vent as close to the coil entry to the cylinder as possible otherwise one would have to slacken/break the connection and pull the pipe back very slightly to check/release any air, however if the coil is circulating and heating the cylinder with both HW & CH on then unlikely as lukewarm water is normally a sign of air.

No difference tonight; in fact possibly worse.
Pump runs very quietly and water always piping hot but there are signs that there could be an air lock or at least a failure to automatically bleed effectively.
Please see photo of bleed stack that runs off feed pipe from boiler to tank. Only gets warm at the base and almost cold at the top. Took another couple of photos - it's a Flamco Flexvent automatic bleed valve. Any way to check operation/manually bleed without physically unbolting, thanks?

 

[John.g]

Can you try running with HW+CH and see if boiler is stable and if the HW tank heats up?
Also ensure boiler pressure gauge 1.5 bar.

Vent: not familiar, others may advise.

 

[rhino666]

Can you try running with HW+CH and see if boiler is stable and if the HW tank heats up?
Also ensure boiler pressure gauge 1.5 bar.

Vent: not familiar, others may advise.

The pressure guage on my system is remote from the boiler, fitted under the expansion vessel. I have always kept it at 1.2 bar cold.
Undid auto bleed valve and seems to have water underneath it. My idea that there may be an airlock in the stack appears wrong. My idea that water heat should rise up the stack appears incorrect; perhaps no potential for circulation in the stack.
Boiler with cental heating and hot water runs exactly as expected; constant burn with temperature gradually rising. I turned the CH off and the Hot water issue returned. Running out of ideas with this now; perhaps I just have to put up with the 15 mins or so of cycling when Hot Water heating only engaged.
Appreciate the feedback on this thread; just a shame I haven't found the solution to this.
Getting back to the fundamental problem. It appears that the boiler is firing up and running on full power on the Hot water circuit, heating up the limited circulation volume too quickly for application. Perhaps the problem is simply that the boiler is not regulating itself correctly? Would have expected the replacement main PCB to remedy that but it was a refurbished board so perhaps the same issue as the original.

 

[John.g]

A few thoughts, all gas boilers fire up at relatively high rates, (I think) up to ~ 70% of output but should then modulate down very rapidly, they generally fire up at the SP-5C and will cut out at SP+5C so if the demand is < 5.5kw (your boiler minimum) the boiler will cycle on/off but not like shown in your post. One of the reasons I asked you to drain down your cylinder until cold is that the coil requirement should ensure that the boiler will run continuously for 20 minutes if everything is "normal" and then start cycling.
Another test (with CH only on) would be to shut off all rads except 1 on say your downstairs circuit, turn up the room stat and see will the boiler cycle normally on this, if the boiler acts up similarly as originally, then IMO it may point to some modulation or other boiler problem.
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Just looking at your readings in post #11 I notice that the boiler fires up again ~ every 30 secs or so after cut out, Afaik all gas boilers have a anti rapid cycling time of around 3 mins and also most have a pump overrun to cool down the heat exchanger, this also lowers the return temperature so when the boiler fires up it has modulated down before it reaches its cut out temperature, one way or the other that doesn't look like normal operation to me, someone familiar with Glowworm boilers might like to comment.